1. Field of the Invention
The present invention relates to amphiphilic nanoscale particles, to processes for their production and to their use.
2. Discussion of Background Information
The production of nanoscale particles usable in industrial processes has long been an interesting objective. When nanoparticles are produced by means of sol-gel or other precipitation techniques, agglomeration can be prevented by applying a surface charge (double-charge cloud). The repulsive forces can be adjusted via the size of the surface charge (ζ potential), so that they prevent coagulation of the particles. The ζ potential can generally be adjusted by the suitable selection of the pH. These connections were identified by Stem as early as the start of the 20th century. The colloid stability achieved as a result leads to the ability to keep nanoparticulate suspensions, also known as sols, stable over prolonged periods.
Another form of stabilization is that of the additional use of functional groups which can likewise bear charges and can be employed to form stable systems via the ζ potential. This is referred to as electrosteric stabilization. The charge-stabilized nanosuspensions or sols can thus be destabilized at the so-called isoelectric point, i.e. a pH is selected at which the surface charge is zero. Aggregates or else gels are then formed if the aggregates combine to larger units.
In addition to these stabilization forms by means of the application of charges, there is yet a further form in which the surface of the nanoparticles is provided with groups which are very similar to the surrounding liquid. This can be achieved, for example, with SiO2 particles whose surface is covered fully with OH groups. The result of this is that, even at the isoelectric point, silicas, for example in aqueous systems, do not precipitate out because the free interface enthalpy between the surface and the surrounding liquid itself is already at a minimum and no enthalpy gain occurs by aggregation. Such stabilizations are typical mainly for aqueous silica sols (SiOH groups).
The avoidance of agglomerates or aggregates is of crucial significance for the further industrial processing of the nanoparticles. This applies both to the production of shaped bodies from such particles and to their dispersion in a matrix material. The interesting properties of nanoparticles, for example interface effects or transparency, can only come into full effect when they are homogeneously dispersible.
While, in the case of electrostatic stabilization (without additional groups), especially in the case of oxidic but also in the case of nonoxidic systems, strong interaction via the formation of chemical bonds (for example SiOSi bonds, significantly stronger in the case of TiOTi or ZrOZr) occurs in the case of formation of aggregates, which enables redispersibility, for example, only with use of strong acids usually undesired in process technology, it is possible in the case of surface modifications with functional groups which do not react chemically with one another for aggregates to occur under certain conditions, which, however, can be returned back to the starting particles in an easily reversible manner with changed conditions.
This principle of chemical surface modification is described in the literature and is frequently used to increase redispersibility. Such surface modification has been described, for example, by the applicant, for example in WO 93/21127 (DE 4212633) or WO 96/31572. However, the disadvantage of this surface modification is that a complicated chemical step is needed for this purpose, in which the surface-modifying molecule has to be converted to a stable bond with groups on the surface of the particles.
According to the prior art, such a surface modification requires heating, for example heating under reflux, and/or the action of shear forces, for example by means of kneaders or mills. An additional factor is that this surface modification virtually has to be tailored to the particular dispersion medium. For example, hydrophilically surface-modified nanoparticles can be dispersed in aqueous or water-like solvents but not in nonpolar solvents, and vice versa.
The object of the present invention was thus directed to the development of nanoparticles for which such surface modification is not required, but which, without further process steps, can be dispersed either in organic solvents or in aqueous solvents. The invention shall additionally solve the problem of significantly reducing the costs for the production of nanoparticles, by virtue firstly of the surface modification as such being unnecessary, and also adjustment and dependency of the dispersion medium required in each case by different surface modifiers no longer being required.